Difficulties are often encountered in pneumatically conveying material such as finely powdered substances which tend to form deposits on the wall of a conduit through which the substances are conveyed.
Such a tendency is shown by particularly fine bulk materials having grain sizes of less than about 10 .mu.m, and occasionally the conduit may become so incrusted with material on its internal surface that the conduit is completely blocked.
Various possible ways of pneumatically conveying such materials have already been proposed, for example in one method of pneumatic conveying, a pipe is arranged within a conveyor conduit and air is blown into the pipe and thence into the conveyor conduit, in order to prevent material from accumulating in the conveyor conduit. However, this is often ineffective when conveying very fine substances such as dye pigments, milk and polyvinylchloride powders. Another method of pneumatically conveying fine materials comprises disposing a by-pass pipe parallel to the conveyor conduit, so that air in the by-pass pipe can be blown into the conveyor conduit to prevent material accumulating. However, the by-pass pipe can easily become blocked.
Another such method comprises introducing material into a conveyor conduit, with an intermittent feed, so that in effect short blocks or plugs of material are conveyed in succession through the conduit. If this method is employed in a conveyor conduit of transparent material, it is possible to see that the blocks or plugs of material are only maintained as such for about 5 to 15 minutes, and then break up into a semicontinuous stream or hank of material, which subsequently breaks up uncontrolledly, resulting in deposits of material being formed on the conduit wall which in turn can result in complete blockage of the conduit.
Yet again, intermediate cleaning of the conveyor conduit, of deposits in the conduit, can be effected by utilising the flow of conveyor air. In this method the delivery of material into the conveyor conduit is temporarily restricted or completely stopped, whenever the conveying pressure in the conduit rises beyong a given value, such rise indicating the danger of a blockage. However, when conveying some substances, or when conveying substances in a certain condition, for example with a high moisture content, the rise in conveying pressure is followed immediately and irresistibly by blockage of the conduit, so that this method is not satisfactory. This applies particularly when the material being conveyed is liable to form on the conduit walls a deposit which subsequently peels off, giving rise to a sudden accumulation of material so that the rise in conveying pressure and blockage of the conduit occur almost simultaneously.